Grain refining is the only strengthening mechanism in structural steels that increases both yield strength and toughness. This concept have been applied successfully in microalloyed steels, in which the use of thermomechanical treatmens and the addition of small quantities of microalloying elements lead to a very fine microstructure that provides a great combination of properties. Microalloying in sintered steels have not been fully explored since it involves some difficulties, like the absence of thermomechanical treatments in a conventional sintering process, the high oxygen affinity of the microalloying elements or the need to ensure a good distribution of the microalloying elements in such small quantity. Besides, the sintering process itself, as a diffusive process, requires temperature and time enough to consolidate properly the material, leading to a quite large microstructure. This work studies the development of new high strength sintered steels alloyed with niobium, and applies the concept of grain refining implemented in microalloyed steels. Niobium is the microalloying element with a higher potential to control the grain growth of steels. In microalloyed steels, niobium have proven to inhibit the grain growth and the recrystallization of austenite in the form of carbides or carbonitrides by particle pinning. But also in solid solution is one of the alloying elements of steels that provides a stronger effect in the grain boundary mobility by solute or impurity drag. Mechanical alloying allows to distribute uniformly the alloying elements in a nanostructured iron powder. Through this technique niobium can be incorporated in elemental form or directly as niobium carbide, providing the possibility to compare both mechanisms to control grain growth (particle pinning and solute drag). In addition to that, the use of special consolidation techniques, like pressure assisted sintering (hot pressing, SPS), allows to reduce the sintering temperatures and times, which helps to moderate the grain growth during sintering. This thesis involves the whole process of design of a new material, from the choice of the composition and the production of the prealloyed powder by mechanical alloying, to the evaluation and validation of the sintered materials.